Histopathology of Seed-Borne Infections - Applied Research Center ...

188 Histopathology of Seed-Borne InfectionsRathayibacter iranicus produces profuse honey-yellow ooze in wheat spikeletsin Iran (Scharif, 1961). The ooze develops from ovaries resulting in abortion ofgrains. No histopathological studies have been made on this pathogen, but its generalbehavior is comparable to that of R. tritici.Rathayibacter rathayi is also transferred by a nematode, Anguina agrostis, andcauses yellow slime disease or Rathay’s disease in Dactylis glomerata and Festucarubra var. fallax (Neergaard, 1979). In these hosts, the bacterium produces profuseyellow slime, which may cover the uppermost leaves, stem, and parts of inflorescence.Contaminated seeds have a varnish-like crust of bacteria.Rathayibacter rathayi and A. agrostis complex cause gall formation in the placeof seeds in rye grass (Lolium rigidum). The presence of nematodes is consideredessential for the gall initiation, but mature galls are colonized predominantly byeither nematodes or bacteria. The two types of galls are almost of the same shapeand size, but differ in color and contents (Stynes et al., 1979; Bird, Stynes, andThompson, 1980). The bacterial galls are bright yellow in comparison to dark brownfor galls containing nematodes. The wall of bacterial galls is thin, and the cavity ispacked with bacterial cells (Figure 6.9 A to C).C. michiganensis subsp. insidiosum, which causes bacterial wilt of alfalfa, occursin the vascular system of the flowering rachis (peduncle) and pedicel, and in vascularand parenchymatous tissues in seed pods. Masses of bacterium were present in thevicinity of aborted seeds. The bacterial cells were seen in the intercellular spacesbelow the malpighian cells in immature seeds, but these were rare in mature seeds(Cormack and Moffatt, 1961).C. michiganensis subsp. michiganensis, causing bacterial canker of tomato, is awell-known vascular invader (Pine, Grogan, and Hewitt, 1955). Detailed histopathologyof infected seed has not been worked out. However, the available informationreveals that C. m. subsp. michiganensis sometimes forms large bacterial pockets inthe chalazal region and can be seen in the inner cells of the seed coat. It does notinvade the endosperm and embryo (Patino-Mendez, 1964). Similarly C. m. subsp.nebraskensis, causing Goss’s bacterial wilt and leaf blight of corn, could be seen inthe chalazal region, the area between the scutellum and endosperm, and in thevicinity of the embryo of the heavily infected seeds (Schuster, 1972). Biddle, McGee,and Braun (1990), after leaf inoculation of a susceptible corn inbred, detected thebacterium in seeds, ear shanks, and stalks. In seeds the pathogen occurred internallyand externally.McBeath and Adelman (1986) detected C. m. subsp. tessellarius in wheat usinga scanning electron microscope. The clusters of bacterial cells were seen in the seedcoat (pericarp), endosperm, and interface near the embryo in infected grains.6.2.4 CURTOBACTERIUMCurtobacterium flaccumfaciens pv. flaccumfaciens, causing bacterial wilt of beans,can be carried in bean seeds either externally or internally (Zaumeyer, 1932). Accordingto Zaumeyer, C. f. pv. flaccumfaciens is primarily a vascular pathogen and tendsto become systemic. The bacterium is located principally in the seed coat (Zaumeyer,